1. Field of the Invention
The present invention relates to a circuit arrangement for capacitive humidity measurement, including at least one capacitive element to be measured, several switch elements, at least one charge storage element, as well as a control unit. The present invention furthermore relates to a method for operating a circuit arrangement for capacitive humidity measurement.
2. Discussion of Related Art
In connection with various applications, there exists today a requirement for the exact monitoring of defined ambient conditions and, if required, their control within a predetermined range. This can be the case, for example, in connection with modern production processes, in the air-conditioning technology, in the field of motor vehicles, or even with household appliances. Besides monitoring and regulating the temperature as the most important ambient parameter, it has become increasingly necessary to also monitor and, if required, regulate the ambient humidity.
Capacitive humidity sensors are often employed on the part of the sensor, which are mostly produced by thin film technology. Customarily, capacitive humidity sensors include two oppositely located flat metal electrodes, between which a hygroscopic material, customarily a polymer, is arranged. The polymer has a humidity-dependent dielectric constant, so that in accordance with measuring technology a plate capacitor results in the end, whose capacitance changes as a function of the relative ambient humidity. Therefore the respective actual capacitance of the humidity sensor functions as the measuring value of the relative humidity. Various measuring methods are used for measuring the capacitance of capacitive humidity sensors.
For example, it is known from DE 41 03 433 A1 to base the capacitance measurement on a frequency measurement in an LC- or RC-oscillating circuit, wherein the measured frequency again changes as a function of the oscillating circuit, and therefore of the relative humidity. Besides the multitude of required structural components for constructing the respective oscillating circuit, as a disadvantage of this variation it must also be noted that it also has a comparatively large electrical current consumption. Added to this is that, for an exact frequency measurement, which does not change even under fluctuating ambient conditions, nor over the course of the operating time, a stable time base is required. These requirements cannot be assured in a simple manner.
Alternatively to this it is known to perform the required capacitance measurement in a capacitive humidity sensor by means of a time measurement. An appropriate circuit is known, for example, from the Application Report SLAA071 (October 1999) of Texas Instruments with the title “Economic Measuring Techniques with a Comparator-A Module (L. Bierl), p. 26 (item 2.7). The charging time required for charging the measuring capacitors to a predetermined voltage is determined by such a process. For measuring small capacitances (C <200 pF) in particular, however, a high chronological resolution of the measurement is required. If the required high resolution of the time measurement is not available, very large resistance values (>1 MΩ) are alternatively required in the respective circuit. In turn, this has negative effects on the stability of the circuit in respect to fluctuating soiling or humidity. Moreover, such interferences can lead to false measurements, since in the course of the measurement a small charge actually is moved only once. If a possible interference results at exactly that time, an erroneous capacitance determination can be the result of this.